With today's rapid development of electronic products, coil has been almost applied to every field, as an essential electronic device. For example, SMD (Surface Mounting Devices) coils are widely used in products like common mode filters, high frequency transformer, impedance transformers, balanced and unbalance transformers, devices for restraining electromagnetic interference (EMI), USB circuits of personal computer and peripheral equipment, liquid crystal display panel, low voltage differential signal devices and car remote keys; closed magnetic circuit high current surface mounting power inductors are widely used in products like video cassette recorder power supplies, LCD TVs, laptops, office automation equipments, mobile communication equipments and AC/DC Converter; RF inductors are widely used in products like mobile telephones, VCO or TCXO circuit and RF transceiver modules, global positioning system, blue tooth modules, communication equipments, LCD TVs, cameras, notebook computers, inkjet printers, photocopier, display monitors, game machines, color televisions, video cassette recorders, optical drives, digital cameras and automotive electronics products; fixed inductance coils are widely used in network, telecommunications, computers, alternating current power supplies and peripheral equipments. Particularly, the fixed inductance coil is most common and most widely used.
Meanwhile, with the development of miniaturization and integration in the electronic industry, requirements on volumes occupied by various electronic devices become higher and higher. However, since the inner diameter of a coil in the prior art is uniform (as shown in FIG. 1), it is apparently unable to achieve an optimized use of a space to be occupied by the coil when a product with the space is to be designed in a special shape, which restricts the performance of the coil and does not contribute to the development of miniaturization of electronic products.
In addition, researches show that the performance of the fixed inductance coil such as impedance, sensitivity, linear range and electromotive force generated from it, has a close relationship with the inner diameter of the coil, namely, the size of the inner diameter of the coil directly affect the performance of the fixed inductance coil. During the production manufacturing of electronic products, a fixed electronic product may need coils with different performances to meet the requirement of function variety, or a coil needs to be changed to a coil with a different performance to meet the upgrade of the product. In this case, for a coil with an uniform inner diameter in the prior art, it needs to replace the whole coil or even abandon the whole electronic product, which will undoubtedly make a big waste of materials and bring an increase in production cost.